The epithelial cells lining the lumenal side of the GIT are a major barrier to drug delivery following oral administration. However, there are four recognized transport pathways which can be exploited to facilitate drug delivery and transport: the transcellular, paracellular, carrier-mediated and transcytotic transport pathways. The ability of a conventional drug, peptide, protein, macromolecule or nano- or microparticulate system to "interact" with one of these transport pathways may result in increased delivery of that drug or particle from the GIT to the underlying circulation.
In the case of the receptor-mediated, carrier-mediated or transcytotic transport pathways, some of the "uptake" signals have been identified. These signals include, inter alia, folic acid, which interacts with the folate receptor, mannose and cetylmannoside, which interact with the mannose receptor, and cobalamin, which interacts with Intrinsic Factor. In addition, leucine- and tyrosine-based peptide sorting motifs or internalization sequences exist, such as YSKV, FPHL, YRGV, YQTI, TEQF, TEVM, TSAF, YTRF, which facilitate uptake or targeting of proteins from the plasma membrane to endosomes. Phage display libraries can be screened using specific membrane receptors or binding sites to identify peptides that bind specifically to the receptor or binding site. The ability of certain motifs or domains of peptides or proteins to interact with specific membrane receptors, followed by cellular uptake of the protein:receptor complex may point towards the potential application of such motifs in facilitating the delivery of drugs. However, the identification of peptides or peptide motifs by their ability to interact with specific receptor sites or carrier sites, such as sites expressed on the apical side of the epithelial sites of the GIT, may not be able to determine, or may not be the most effective way to determine, the identity of peptides capable of enhancing the transport of an active agent, especially a drug-loaded nano- or microparticle, through tissues such as epithelial lining.
Non-receptor-based assays to discover particular ligands have also been used. For instance, a strategy for identifying peptides that alter cellular function by scanning whole cells with phage display libraries is disclosed in Fong et al., Drug Development Research 33:64-70 (1994). However, because whole cells, rather than intact tissue or polarized cell cultures, are used for screening phage display libraries, this procedure does not provide information regarding sequences whose primary function includes affecting transport across polarized cell layers.
Additionally, Stevenson et al., Pharmaceutical Res. 12(9), S94 (1995) discloses the use of Caco-2 monolayers to screen a synthetic tripeptide combinatorial library for information relating to the permeability of di- and tri-peptides. While useful, this technique does not assess the ability of the disclosed di- and tri-peptides to enhance delivery of a drug, especially a drug-loaded nano-or microparticle formulation.
Thus, there exists a need for a method of determining peptide sequences that are particularly effective in transporting drugs, including drug-loaded nano- and microparticles, across a human or animal tissue barrier.